[Chemistry Class Notes] on Viscosity and Surface Tension Pdf for Exam

Surface tension Viscosity is dependent on molecular interactions.

• Viscosity occurs because of an alliance between identical molecules located in the same liquid material.

• Surface tension is determined by the difference in interactions between liquid molecules with the molecules of the liquid in contact.

Introduction

A fluid is a substance that has no fixed shape. Liquids and Gases can flow easily and are termed as fluids. In this topic, we will learn about two Mechanical Properties of Fluids which are Viscosity and Surface Tension. Both of them are dependent on molecular interactions. The viscosity of a fluid is a measure of how sticky the fluid is, on the other hand, Surface tension is the elastic property of a fluid which makes it acquire the least surface area possible. Now let’s discuss each of them.

What is Surface Tension?

Surface tension is defined as the tendency of a fluid to shrink into the minimum possible surface area. This property of liquid is based on the fact that the molecules of the liquid at the surface level are in a different situation than those in the interior of the liquid. The molecule lying inside the liquid surface is surrounded by other molecules and is attracted equally in all directions. So, the net force of attraction acting on the molecule is zero. Surface tension does not depend on the forces of attraction between the particles only but also depends on the forces of attraction of solid, liquid, or gas in close contact with it.

The energy required for surface tension is equal to the work or energy required to remove the surface layer of the molecules in a unit area.  Surface tension is expressed in energy (joules) per unit area (square meters). It is often expressed as an amount of force exerted on the surface perpendicular to a line of unit length. Water has a surface tension of 0.07275 joules per square meter at 20 °C.

While other organic liquids, such as benzene and alcohols have lower surface tensions, whereas mercury has higher surface tension. Increasing the temperature lowers the net force of attraction between the molecules and decreases surface tension. The most common example we can observe in our daily life is when we add soap to the water, its surface tension decreases, thereby the liquid with soap mixes easily with dirt on hand and cleans it.

Surface tension releases the energy required to increase the surface of the liquid by unit area. Also, surface tension is also the fundamental property of the liquid surface that holds out against force. Especially it keeps a hurdle between foreign materials and liquid as well as this is the force that holds the liquid molecules bound together.

Another example, The drops of a liquid are spherical because, for a given volume, a sphere has a minimum surface area. (Surface tension tries to acquire the minimum surface area of the liquid). The driving forces behind the origin of surface tension are cohesive and adhesive forces.

What is Viscosity?

Viscosity can be defined as the property of a fluid by virtue of which it opposes the relative motion between two surfaces of the fluid that are moving at different velocities or in simple terms, viscosity means friction between the molecules of fluid or opposition to flow. The liquids which flow fast, have lower internal resistance and vice versa.

This is because of the strong intermolecular forces between the molecules. These liquids are less viscous and have low viscosity. The liquids which flow very slow have high internal resistance. This is because of the weak intermolecular forces between them. So, they are less viscous or have low viscosity.

For Viscosity of fluids, the tangential, or shearing, stress that causes flow is directly proportional to the rate of shear strain, or rate of deformation, the shear stress divided by the rate of shear strain is constant for a given fluid at a given temperature. The constant is termed as the absolute, dynamic, or simply called viscosity, Fluids that behave in this way are called Newtonian fluids, The unit of viscosity, is newton-second per square meter, which is usually expressed as pascal-second in SI units.

The viscosity of liquids is decreased when the temperature is increased, and the viscosity of gases is increased when the temperature increases. Thus, upon heating, liquids will flow more easily, whereas gases flow more sluggishly. In some cases, Viscosity is also known as frictional forces that act between fluid and the surface of contact. The surface may be solid like a pipe, and water will be the fluid.

The resistance provided by the pipe to the water flow is considered as Viscosity. However, Viscosity can also be created between two liquids present at different velocities. Eg: Oil has a high viscosity, but when we put it in the car and heat it up, it becomes less viscous and thus flows smoothly through an engine and other parts of the car. The higher the viscosity, the slower the liquid flow.

 

Conclusion

The above description of Viscosity is very much particular to liquid, to be accurate Newtonian liquids. On the other hand, in the case of non-Newtonian liquid, The interactions are pretty bizarre and are always hard to model and understand.

Viscosity and surface tension is dependent on some situation like temperature increases. Viscosity and surface tension both change with the changing of temperature. Viscosity and surface tension both are essential properties for liquids. There is a relationship between them that surface tension varies directly as Viscosity at a constant temperature.

 

However, viscosity and surface tension also depend on the liquid’s material, area, and other environmental issues. It is an essential physics topic applicable in both whole and research life. This topic is also essential to creating new things.